Cow pus. It all started with cow pus, nearly 220 years ago. For more on vaccines: http://www.vox.com/2015/2/10/8007571/vaccine-myths Subscribe to our channel! http://goo.gl/0bsAjO Vox.com is news website that helps you cut through the noise and understand what's really driving the events in the headlines. Check out http://www.vox.com/ to get up to speed on everything from Kurdistan to the Kim Kardashian app. Check out our full video catalog: http://goo.gl/IZONyE Follow Vox on Twitter: http://goo.gl/XFrZ5H Or on Facebook: http://goo.gl/U2g06o Credit: Words by: Julia Belluz Produced by: Estelle Caswell